High-thermal-mass hydronic furnace

ABSTRACT

Various embodiments of the present invention are directed to a high-thermal-mass hydronic furnace. In one embodiment of the present invention, a high-thermal-mass hydronic furnace includes a firebox, an insulated outer casing, a fluid-transport system, and a draft air-flow system. Fuel input to the firebox is combusted on a high-thermal-mass ceramic refractory. Heat from the fuel combustion is transferred to fluid within the fluid-transport system, via an internal heat exchanger, and circulated to a location external to the insulated outer casing for subsequent distribution to an interconnected heat-delivery system. The draft air-flow system regulates the fuel combustion by controlling the amount of air passing through the firebox.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Application No. 60/810,609, filed Jun. 2, 2006.

TECHNICAL FIELD

The present invention relates to the field of hydronic furnaces, and, in particular, to a high-thermal-mass hydronic furnace.

BACKGROUND OF THE INVENTION

For many years, wood-fired furnaces have been used as a relatively simple and inexpensive heat source for residential, commercial, and industrial buildings (“buildings”) of various sizes. Wood-fired furnaces may either be used as a sole heat source, or as a supplemental heat source for other sources of heat, such as oil, natural gas, or electricity. One common type of wood-fired furnace is a hydronic furnace. Hydronic furnaces use a fuel to heat a heat-transfer fluid (“fluid”) that is distributed throughout an area to be heated. For example, water may be heated and distributed to selected radiators located throughout a house.

One type of hydronic furnace is an outdoor wood-fired boiler (“OWB”). An OWB is often a self-standing structure placed within several hundred feet of one or more buildings to be heated. Typically, an OWB is interconnected to each of the buildings to be heated by a number of insulated pipes. FIG. 1 shows a front view of an OWB providing heat for a nearby house. In FIG. 1, an OWB 100 is shown providing heat for a nearby house 102. The OWB 100 includes a combustion chamber 104 with a loading door 106 and a water tank 108 surrounding the combustion chamber 104. Wood 110 may be burned in the combustion chamber 104 to heat water in the surrounding water tank 108. Air is often input to the combustion chamber 104 with the aid of motorized fan (not shown in FIG. 1) forcing the air into the combustion chamber 104. Exhaust 112 is output from an air output 114. The heated water in the water tank 108 is passed to and from the house 102, via insulated pipes 116, beneath ground level 118. The heated water passes in the directions shown by directional arrows, such as directional arrow 120. The movement of the water may be aided by one or more circulating pumps, such as circulating pumps 122. Energy, in the form of heat in water, may be transferred to a heat-distribution system in the house 102, such as a series of radiators, represented in FIG. 1 as a dashed rectangle 124. The heated water may also be interconnected to a hot water supply in the house 102, represented in FIG. 1 as a dashed cylinder 126.

An OWB may be an attractive heating system for some people. In areas where wood is plentiful, an OWB may be a less expensive heating system than heating systems using oil, natural gas, or electricity. Additionally, an OWB may be manufactured with variable-sized combustion chambers in order to accommodate the heating needs of various numbers and sizes of buildings, and to regulate how often fuel needs to be added to a combustion chamber. However, an OWB may also have several drawbacks. Combustion chambers are typically fabricated from steel. A surrounding water tank prevents temperatures in a combustion chamber from reaching the temperatures necessary to completely combust input wood. Consequently, particulates, such as smoke and creosote (“emissions”), are produced during the combustion process and are copiously output from an OWB. Emissions from an OWB sometimes exceed allowable limits in some municipalities and may cause unhealthy, toxic air conditions, as well as unsafe visibility levels. Consequently, a growing number of municipalities have banned the use of OWBs at current emission levels.

In response to developing emission restrictions in certain municipalities, some manufacturers of wood-fired furnaces have introduced systems that use catalytic technologies in an effort to reduce emissions. In a catalyst-equipped wood-fired furnace, exhaust is passed through a ceramic honeycomb element coated with platinum or palladium. Although catalyst-equipped wood-fired furnaces may reduce emissions to levels that are deemed acceptable in many municipalities, a resulting loss in thermal efficiency often results. A loss in thermal efficiency often results in an increase in usage cost. Wood-fired-hydronic-furnace manufacturers, distributors, sellers, as well as people desiring to heat one or more buildings have, therefore, recognized a need for an efficient hydronic furnace that is inexpensive to operate and creates emissions at or below governmentally-acceptable levels.

SUMMARY OF THE INVENTION

Various embodiments of the present invention are directed to a high-thermal-mass hydronic furnace. In one embodiment of the present invention, a high-thermal-mass hydronic furnace includes a firebox, an insulated outer casing, a fluid-transport system, and a draft air-flow system. Fuel input to the firebox is combusted on a high-thermal-mass ceramic refractory. Heat from the fuel combustion is transferred to fluid within the fluid-transport system, via an internal heat exchanger, and circulated to a location external to the insulated outer casing for subsequent distribution to an interconnected heat-delivery system. The draft air-flow system regulates the fuel combustion by controlling the amount of air passing through the firebox.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a front view of an OWB providing heat for a nearby house.

FIG. 2A shows a front perspective view of a furnace that represents one embodiment of the present invention.

FIG. 2B shows a rear perspective view of the furnace shown in FIG. 2A that represents one embodiment of the present invention.

FIG. 3 shows a rear perspective view of the furnace shown in FIG. 2A without an outer casing that represents one embodiment of the present invention.

FIG. 4A shows a close-up view of an open damper over an air intake system for the furnace shown in FIG. 2A that represents one embodiment of the present invention.

FIG. 4B shows a close-up view of a closed damper over an air intake system for the furnace shown in FIG. 2A that represents one embodiment of the present invention.

FIG. 5A shows a front perspective view of a one-piece high-thermal-mass ceramic refractory for a furnace that represents one embodiment of the present invention.

FIG. 5B shows a rear perspective view of the one-piece high-thermal-mass ceramic refractory shown in FIG. 5A for a furnace that represents one embodiment of the present invention.

FIG. 5C shows an exploded view of a multiple-piece high-thermal-mass ceramic refractory for a furnace that represents one embodiment of the present invention.

FIG. 6 shows a side view of a fluid-transport system for the furnace shown in FIG. 2A that represents one embodiment of the present invention.

FIG. 7 shows the movement of air during operation of the furnace shown in FIG. 2A that represents one embodiment of the present invention.

FIG. 8 shows the movement of fluid in the fluid-transport system during operation of the furnace shown in FIG. 2A that represents one embodiment of the present invention.

FIG. 9 shows a schematic view of a heat-distribution system interconnected with the furnace shown in FIG. 2A that represents one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the present invention are directed to a high-thermal-mass hydronic furnace. In one embodiment of the present invention, the high-thermal-mass hydronic furnace (“furnace”) cleanly combusts wood input to a crucible-shaped, high-thermal-mass ceramic refractory in a firebox. Heat produced during the combustion is efficiently transferred to circulating fluid within a fluid-transport system. The heat contained in the fluid may then be distributed to a heat-delivery system. FIG. 2A shows a front perspective view of a furnace that represents one embodiment of the present invention. In FIG. 2, a furnace 200 includes an outer casing 202 and a firebox door 204 surrounding a loading aperture 205 on a front face 206 of the outer casing 202. A firebox 208 is positioned inside the loading aperture 205. FIG. 2B shows a rear perspective view of the furnace shown in FIG. 2A that represents one embodiment of the present invention. In FIG. 2B, the outer casing 202 includes a top face 210 and a rear face 212. An external portion 213 of a fluid-transport system 214 extends from the top face 210 to the rear face 212. An expansion tank 216 interconnects with the fluid-transport system 214 via an expansion-tank line 218. Note that only the external portion 213 of the fluid-transport system 214 is shown in FIG. 2B. Additional portions of the fluid-transport system 214 are inside the outer casing 202 and are described below, with reference to FIG. 3 and FIG. 6. The rear face 212 includes a draft air-flow system 220. The draft air-flow system 220 includes an exhaust vent 221 and an air intake system 222. The exhaust vent 221 and the air intake system 222 are both interconnected to the firebox (208 in FIG. 2A). In one embodiment of the present invention, air entering the firebox (208 in FIG. 2A) passes through the air intake system 222 and air exiting the firebox (208 in FIG. 2A) passes through the exhaust vent 221. A damper actuator 224 may be used to control the amount of air input to the air intake system 222 by adjusting a damper 226. In FIG. 2B, the damper 226 is shown in a closed position so that a relatively small amount of air is input to the firebox (208 in FIG. 2A).

FIG. 3 shows a rear perspective view of the furnace shown in FIG. 2A without an outer casing that represents one embodiment of the present invention. The furnace 200 includes the firebox 208, the fluid-transport system 214, and the draft air-flow system 220. The firebox 208 includes a refractory 302 for promoting the attainment of high temperatures during the combustion of input wood 304 and for storing generated heat. Note that, in FIG. 3 and in later figures, one of the side walls of the refractory 302 is omitted to show the interior of the crucible-shaped refractory 302. The fluid-transport system 214 includes the external portion 213, shown in FIG. 2B, and an internal portion 306. When the outer casing (202 in FIG. 2A) is fitted onto the furnace 200, the internal portion 306 of the fluid-transport system 214 is contained within the outer casing (202 in FIG. 2A) and, as shown in FIG. 2B, the external portion 213 of the fluid-transport system 214 is external to the outer casing (202 in FIG. 2A).

The external portion 213 of the fluid-transport system 214 includes a first aquastat 308 and a second aquastat 310. The first aquastat 308 controls the furnace 200 by controlling the air flow into the firebox 208, via the draft air-flow system 220. The first aquastat 308 monitors the temperature of fluid contained within the fluid-transport system 214 and controls the damper (226 in FIG. 2A), via the damper actuator 224. The use of the second aquastat 310 may vary depending on the particular needs of the furnace user. The second aquastat 310 may be used to regulate heat dissipation when the heat from the combusting furnace 200 is not currently desired. When the furnace 200 is interconnected with a secondary heating system, the second aquastat 310 may also be used to interface with the secondary heating system to coordinate usage between the furnace 200 and the secondary heating system. When an interconnected secondary heating system is being used, the second aquastat 310 may be configured to respond to prompts made from a controller or a relay associated with the interconnected secondary heating system.

FIG. 4A shows a close-up view of an open damper over an air intake system for the furnace shown in FIG. 2A that represents one embodiment of the present invention. The draft air-flow system 220 includes an exhaust vent 221 and an air intake system 222. The exhaust vent 221 may be interconnected to an existing flue in a building or a new flue may be built. Depending on the conditions in the location of the furnace and building containing the furnace, a draft occurs in an attached flue due to changes in pressure. The pressure at the exhaust vent 221 is generally lower than the pressure at the top of an attached flue. The change is pressure causes air to exit the exhaust vent 221 and pass up the flue. The low pressure at the exhaust vent 221 also causes air to be drawn into the furnace through the air intake system 222. Note that when, on occasion, an insufficient or negative draft occurs, an inducer fan may be attached to an interconnected flue to induce a draft.

The air intake system 222 includes a number of air-input apertures, such as air-input aperture 402, the damper 226, and the damper actuator 224. The air-input apertures extend through the rear face 212 of the outer casing 202, through the refractory (not shown in FIG. 4A), and into the firebox (not shown in FIG. 4A). The damper 226 is positioned over each of the air-input apertures. In FIG. 4A, the damper 226 is open to maximize the amount of air passing into the firebox (not shown in FIG. 4A). When a user of the furnace 200 desires to reduce the amount of heat output from the furnace 200, the user may select a lower temperature via the first aquastat (not shown in FIG. 4A) or via a thermostat or some other temperature controller on an interconnected secondary heating system. The first aquastat (not shown in FIG. 4A) passes a signal to the damper actuator 224 to close the damper 226 so that less air is input to the firebox (not shown in FIG. 4A) and consequently, the amount of heat output from the furnace 200 is reduced. FIG. 4B shows a close-up view of a closed damper over an air intake system for the furnace shown in FIG. 2A that represents one embodiment of the present invention. In alternate embodiments of the present invention, a first aquastat may instruct a damper motor to make incremental adjustments to a damper to finely adjust air flow into a firebox.

A firebox is where a fuel source, such as wood, is combusted to create heat. When wood ignites, the temperature may accelerate to a temperature of approximately 500° F., at which point the wood begins to breakdown chemically and emit gases. The emitted gases may combust causing the temperature to accelerate to approximately 110° F., at which point the solid wood begins to combust. When the combustion is able to continue at a temperature at or above 110° F., a complete combustion may occur. In a complete combustion, the combustion may continue until all of the solid wood and the emitted gases are consumed. When the combustion is unable to continue at a temperature at or above 1100° F., an incomplete combustion may occur. In an incomplete combustion, unconsumed solid wood and emitted gases may be vented by a furnace as one or more types of particulates, such as smoke and creosote. In an OWB, the surrounding water tank prevents the combustion chamber from sustaining a temperature high enough for complete combustion to occur. Thus, a relatively large amount of emissions may be output from an OWB. Conversely, in one embodiment of the present invention, a firebox for a furnace contains a ceramic refractory with a mass of at least 140 pounds per cubic foot of firebox, or a “high-thermal-mass ceramic refractory,” that is able to withstand sustained temperatures in a firebox at or above temperatures obtained during the combustion of wood and accompanying gases so that a complete combustion may be obtained.

FIG. 5A shows a front perspective view of a one-piece high-thermal-mass ceramic refractory for a furnace that represents one embodiment of the present invention. A one-piece high-thermal-mass ceramic refractory 500 includes a front face 502, a rear face 504, a first side face 506, and a second side face 508. The front face 502 includes a front-face top edge 510, the rear face 504 includes a rear-face top edge 512, the first side face 506 includes a first-side-face top edge 514, and the second side face 508 includes a second-side-face top edge 516. Collectively, the front face 502, the rear face 504, the first side face 506, and the second side face 508 enclose the firebox 518 to form a crucible shape. The front-face top edge 510 is at a level that is lower than the other three top edges so that fuel may be loaded into the one-piece high-thermal-mass ceramic refractory 500 from the front. The rear-face top edge 512 includes a corbel 520 that extends inward over the firebox 518 and is at a level that is higher than the front-face top edge 510 and lower than both the first-side-face top edge 514 and the second-side-face top edge 516. The corbel 520 reflects heat created during fuel combustion to enable the attainment and sustainability of high temperatures. The top edges 510, 512, 514, and 516 are positioned at various levels to direct air flow out of the firebox 518, as discussed below with reference to FIG. 7. Note that the one-piece high-thermal-mass ceramic refractory 500 also includes a bottom face on which fuel combustion occurs.

FIG. 5B shows a rear perspective view of the one-piece high-thermal-mass ceramic refractory shown in FIG. 5A for a furnace that represents one embodiment of the present invention. The rear face 504 includes a number of air-input apertures, such as air-input aperture 522. The air-input apertures are part of the air intake system (222 in FIG. 4A) discussed above, with reference to FIG. 4A. The air-input apertures allow ample air to enter the firebox 518 to maintain fuel combustion.

FIG. 5C shows an exploded view of a multiple-piece high-thermal-mass ceramic refractory for a furnace that represents one embodiment of the present invention. In FIG. 5C, a multiple-piece high-thermal-mass ceramic refractory 524 includes a separate front face 526, rear face 528, first side face 530, second side face 532, and bottom face 534. Each of the faces 526, 528, 530, 532, and 534 may be fabricated separately and subsequently assembled in a furnace.

In addition to enabling the attainment of sustained high temperatures, a ceramic refractory may also be used to store heat when a furnace is not in use. Heat stored in a high-thermal-mass ceramic refractory may support automatic re-firing of fuel input to a firebox for several days after the previous combustion. Storing heat in a high-thermal-mass ceramic refractory may also be safer than storing heat in a water tank, such as with an OWB, because a high-thermal-mass ceramic refractory does not store heated water in a contained space. Large quantities of heated water may create a pressure build-up that may potentially lead to an explosion when a relief valve fails.

Combustion efficiency is a measure of how well a furnace converts input fuel to useful energy. A high-thermal-mass ceramic refractory promotes an increase in combustion efficiency by enabling the attainment of temperatures high enough to support the complete combustion of input fuel, such as wood. As discussed above, a high-thermal-mass ceramic refractory needs to include at least 140 pounds of ceramic refractory per cubic foot of firebox. Thus, more than one thousand pounds of ceramic refractory are needed for an 8.5 cubic foot firebox. In one embodiment of the present invention an 8.5 cubic foot firebox contains a high-thermal-mass ceramic refractory that is four to six inches thick and weighs 1300 pounds. When a high-thermal-mass ceramic refractory of 1300 pounds is used in an 8.5 cubic foot firebox, tests have shown the attainment of a combustion efficiency of approximately 96%.

Thermal efficiency is a measure of the rate at which heat exchange surfaces transfer heat to a transfer medium, such as from air to water. Thermal efficiency is typically measured as a ratio of British Thermal Unit (“BTU”) output of hot water to BTU input of fuel. The use of high-thermal-mass ceramic refractory may increase thermal efficiency by enabling prolonged high temperatures in a firebox. Additionally, the shape used for the high-thermal-mass ceramic refractory may affect thermal efficiency by channeling air flow, as discussed below, with reference to FIG. 7.

Thermal efficiency may also be affected by minimizing the amount of heat loss. Accordingly, insulation may be used to minimize the amount of heated air passing through an outer casing for a furnace. In one embodiment of the present invention, three different types of insulation are used in various locations around the furnace to minimize heat loss. Ceramic fiber blankets may be used to line an outer casing and be positioned in locations exposed to direct flames. Additionally, several different types of mineral wool may be used to line a firebox to form an insulated layer between a high-thermal-mass ceramic refractory and an outer casing. Insulation may be attached to surfaces using heat-resistant metals, such as welding insulation to an outer casing using iron washers. In one embodiment of the present invention, when a crucible-shaped, high-thermal-mass ceramic refractory is used and a firebox and outer casing are insulated, tests have shown the attainment of a thermal efficiency of approximately 87%. In addition to increasing thermal efficiency by including insulation, safety is increased as well because the outer surfaces of a furnace are maintained at a temperature that typically does not burn an individual touching the outer surface of a furnace.

FIG. 6 shows a side view of a fluid-transport system for the furnace shown in FIG. 2A that represents one embodiment of the present invention. The top face 210 and the rear face 212 of the outer casing 202, shown in FIG. 6 as a double dashed line, separates the internal portion 306 from the external portion 213. The internal portion 306 includes an internal heat exchanger 602. In FIG. 6, the internal heat exchanger 602 is shown as two rows of bent pipes 604 and 606. When the internal heat exchanger 602 is placed over combusting fuel in a firebox, heat from the combustion is transferred to circulating fluid, such as water and propylene glycol, within the internal heat exchanger 602. In one embodiment of the present invention, a fluid-transport system includes fluid with approximately 70% water and 30% propylene glycol. The propylene glycol may be used for ameliorating rust build-up and reducing the chance of the fluid freezing in the fluid transfer system during cold weather when the furnace is not in use.

The external portion 213 of the fluid-transport system 214 includes a temperature gauge 608 for monitoring the temperature of fluid in the fluid-transport system 214, the first aquastat 308 and the second aquastat 310 for controlling operation of the furnace (200 in FIG. 2A), a float vent 610 for dissipating air entrapped in the fluid, a temperature and pressure valve 612, a flat panel heat exchanger 614 for transferring heat from the fluid within the fluid-transport system 214 to fluid within a heat-delivery system, a circulating pump 616 for circulating fluid in the fluid-transport system 214, and a drain valve 618 for releasing fluid from the fluid-transport system 214.

During operation, the fluid-transport system 214 needs to be able to continuously circulate fluids while withstanding temperatures of approximately 2000° F. In one embodiment of the present invention, a fluid-transport system is fabricated from American Society for Testing and Materials Grade A36 mild steel plate and Schedule 40 steel pipe, using a combination of gas metal arc welding, gas tungsten arc welding, laser or waterjet cutting, and precision pipe bending.

FIG. 7 shows the movement of air during operation of the furnace shown in FIG. 2A that represents one embodiment of the present invention. When input wood 304 is ignited in the firebox 208, air from outside the furnace 200 is input through the air intake system 222, as shown by directional arrow 702. As the temperature of the air in the firebox 208 increases, the heated air rises, as shown by directional arrows 704. The heated air passes in proximity to the internal heat exchanger 602, transferring heat to fluid within the fluid-transport system 214. The draft from the draft air-flow system 220 maintains air movement along the length of the fluid-transport system 214 to maximize heat transfer. The air moves horizontally, then downward, and out through the exhaust vent 221, as shown by directional arrows 706, 708, and 710, respectively. The air output from the exhaust vent 221 is in proximity to the air input to the air intake system 222. When the temperature of the air entering the furnace 200 is approximately room temperature (66° F. to 74° F.), the air output from the furnace 200 is at a temperature that is higher than room temperature. Consequently, the output air pre-heats the input air by transferring heat to the air being input to the furnace 200. Pre-heating input air may increase thermal efficiency. In one embodiment of the present invention, the temperature of the air entering an air intake system is at room temperature. The input air is heated to approximately 2000° F. in a firebox and rises to an internal heat exchanger. After traveling the length of the internal heat exchanger, the temperature of the heated air then lowers to approximately 300° F. as the air is output from an exhaust vent.

FIG. 8 shows the movement of fluid in the fluid-transport system during operation of the furnace shown in FIG. 2A that represents one embodiment of the present invention. When air is heated in the firebox 208, the heated air rises to the top of the firebox 208, where the internal heat exchanger 602 is located, and transfers heat to fluid contained within the internal heat exchanger 602. The circulating pump 616 controls the movement of the heated fluid. In FIG. 8, fluid movement is in a clockwise direction through the fluid-transport system 214, as shown by a number of directional arrows placed end-to-end, such as directional arrow 802, within the fluid-transport system 214. When the heated fluid reaches the flat panel heat exchanger 614, the heated fluid flows in proximity to flowing fluid within an interconnected heat-delivery system. Heat from the fluid within the fluid-transport system 214 is transferred to the fluid within the interconnected heat-delivery system. As a result, the temperature of the fluid within the fluid-transport system 214 is lowered as the fluid flows through the flat panel heat exchanger 614. The cooled fluid in the fluid-transport system 214 is then passed in close proximity to the air flowing out the exhaust vent 221. Consequently, the temperature of the fluid in the fluid-transport system 214 may increase before flowing back into the internal heat exchanger 602 to repeat the above-described process. Increasing fluid temperature prior to reaching the internal heat exchanger 602 may increase thermal efficiency.

The interconnected heat delivery system may be a heat-delivery system exclusively for the furnace 200, or the heat-delivery system may be part of an existing heating system to which the furnace is interconnected as one of multiple possible heat sources. In an alternate embodiment of the present invention, a fluid-transport system does not include a flat panel heat exchanger. Instead, a heat-delivery system is a direct part of the fluid-transport system. When a heat-delivery system is included as part of a fluid-transport system, a larger circulating pump may be necessary to accommodate the additional distances traveled by fluids within the fluid-transport system.

The expansion tank 216 is interconnected to the fluid-transport system 214 in proximity to the circulating pump 616. The expansion tank 216 accommodates thermal expansion of fluid within the fluid-transport system 214 and supplies additional fluid to the fluid-transport system 214 when the fluid level falls below a predetermined level. In one embodiment of the present invention, the expansion tank 216 is open to the atmosphere. Thus, the fluid-transport system 214 may avoid becoming pressurized when, for example, an excessive amount of fuel is combusted, the circulating pump fails, the damper actuator fails, and/or the amount of fluid in the fluid-transport system 214 falls below a level needed to circulate fluid.

FIG. 9 shows a schematic view of a heat-distribution system interconnected with the furnace shown in FIG. 2A that represents one embodiment of the present invention. In FIG. 9, the furnace 200 is shown providing heat to an existing hot water heater 902 and an existing boiler 904. A circulating pump 906 circulates heated fluid from the furnace 200 to the existing hot water heater 902 and existing boiler 904. A circulating pump 908 circulates heated water from the existing boiler 904 out to an outgoing portion of a heat-delivery system 910. Once the heat-delivery system 910 has used the heat from the heated fluid, the spent fluid is returned to the furnace 200 via the returning portion of the heat-delivery system 912.

Selective safety measures may be used in connection with operation of a furnace. In one embodiment of the present invention, a fail-safe damper actuator is used to control damper movement. When an interruption to the furnace power supply occurs, the damper is placed in a closed position so that airflow into the firebox is vastly reduced; thereby causing any current combustion to cease when denied an air supply. In another embodiment of the present invention, a low-water cutoff switch is installed that causes the damper actuator to place the damper in a closed position when the level of fluid in the fluid-transport system falls below a predetermined level. In yet another embodiment of the present invention, the probes for the aquastats are positioned in an immersion well that is mounted below the prime level of the fluid in the fluid-transport system so that, in the event of a low fluid level, the aquastat probes remain immersed in fluid.

A furnace may be placed indoors or outdoors. Thus, a furnace may be placed in many possible locations, including a room in a building to be heated, a nearby shed, a garage, a basement, or other location. A flue is often interconnected to an exhaust vent for creating a draft to maximize thermal and combustion efficiency and for relocating emission dissipation to a location away from high-use areas. Accordingly, a furnace may be positioned in or around an existing chimney. However, a flue may be built specifically for a furnace at other desired locations.

Additional modifications within the spirit of the invention will be apparent to those skilled in the art. For example, the size and shape of many individual parts may be altered. The size of the firebox may be adjusted to accommodate various amounts and sizes of input wood. The lengths and diameters of various parts of the draft air-flow system may be adjusted to accommodate the creation and maintenance of a draft for various sizes of fireboxes and for the combustion of various types of fuel sources. The lengths and diameters of the fluid-transport system may be adjusted to accommodate various sizes of fireboxes and the combustion of various types of fuel sources. Additionally, the flow rate used within a fluid-transport system may be adjusted to improve thermal efficiency and accommodate various heating needs. Various types and amounts of insulation may be used to improve combustion efficiency and improve safety. In various embodiments of the present invention, wood has been used as an example of a combustible fuel source. However, other fuel sources may be used as well, such as various types of biomass, including switchgrass, hemp, corn, poplar, willow, sugarcane, and other types of biomass.

The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. The foregoing descriptions of specific embodiments of the present invention are presented for purpose of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments are shown and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents: 

1. A high-thermal-mass hydronic furnace comprising: an insulated outer casing; a firebox within the insulating outer casing for combusting input fuel, the firebox including a ceramic refractory; a draft air-flow system connecting the firebox to the environment external to the outer casing, the draft air-flow system including an air intake system for bringing air into the firebox and an exhaust vent for removing air from the firebox; and a fluid-transport system interconnecting the firebox to the environment external to the outer casing, the fluid-transport system containing a circulatable heat-transfer fluid.
 2. The high-thermal-mass hydronic furnace of claim 1 wherein the ceramic refractory is crucible-shaped.
 3. The high-thermal-mass hydronic furnace of claim 1 wherein the firebox includes at least one hundred forty pounds of ceramic refractory per cubic foot of firebox.
 4. The high-thermal-mass hydronic furnace of claim 1 wherein the insulation in the outer casing includes one or more of mineral wool; and a ceramic fiber blanket.
 5. The high-thermal-mass hydronic furnace of claim 1 wherein the air intake system includes a number of air-input apertures in the high-thermal-mass ceramic refractory and outer casing; a movable damper positioned over top of the air-input apertures on the outer casing; and a damper actuator for moving the movable damper.
 6. The high-thermal-mass hydronic furnace of claim 5 wherein the fluid-transport system includes a first aquastat for monitoring the temperature of the circulatable heat-transfer fluid and transmitting signals to the damper actuator.
 7. The high-thermal-mass hydronic furnace of claim 5 wherein the movable damper may be placed in one of an open position wherein the air-input apertures are unobstructed; and a closed position wherein the air-input apertures are obstructed.
 8. The high-thermal-mass hydronic furnace of claim 7 wherein the draft air-flow system utilizes a draft to draw air into the firebox when the damper is in an open position and to vent air from the firebox out the exhaust vent.
 9. The high-thermal-mass hydronic furnace of claim 1 wherein the exhaust vent is interconnected to a flue.
 10. The high-thermal-mass hydronic furnace of claim 1 wherein the fluid-transport system includes an internal heat exchanger positioned above the firebox.
 11. The high-thermal-mass hydronic furnace of claim 10 wherein the circulatable heat-transfer fluid in the internal heat exchanger becomes heated in response to the heat produced by fuel combusting in the firebox.
 12. The high-thermal-mass hydronic furnace of claim 1 wherein the fluid-transport system includes a circulating pump for circulating the circulatable heat-transfer fluid in a continuous loop through the fluid-transport system.
 13. The high-thermal-mass hydronic furnace of claim 1 wherein the fluid-transport system includes a flat panel heat exchanger for transferring heat from the circulatable heat-transfer fluid to an interconnected heat-delivery system.
 14. The high-thermal-mass hydronic furnace of claim 1 wherein the fluid-transport system includes a second aquastat interfaced with a second heating source for coordinating usage between the high-thermal-mass hydronic furnace and the second heating source.
 15. The high-thermal-mass hydronic furnace of claim 1 wherein the exhaust vent is positioned in proximity to the air-input apertures for pre-heating air input to the firebox.
 16. The high-thermal-mass hydronic furnace of claim 1 wherein the exhaust vent is positioned in proximity to the fluid transfer system after the circulatable heat-transfer fluid has passed the flat panel heat exchanger, but before the circulatable heat-transfer fluid has reached the internal heat exchanger for pre-heating the circulatable heat-transfer fluid input to the internal heat exchanger.
 17. The high-thermal-mass hydronic furnace of claim 1 wherein the combusted fuel is one of wood; and biomass.
 18. A method for heating a room, the method comprising: providing a high-thermal-mass hydronic furnace, the high-thermal-mass hydronic furnace including a firebox within an insulated outer casing, the firebox having a ceramic refractory, and a fluid-transport system interconnecting the firebox to the environment external to the outer casing, the fluid-transport system containing a circulatable heat-transfer fluid; igniting fuel input to the firebox; transferring the heat from the combusting fuel in the firebox to the circulatable heat-transfer fluid; transferring the heat from the circulatable heat-transfer fluid to an interconnected heat-delivery system.
 19. The method of claim 18 wherein the firebox includes at least one hundred forty pounds of ceramic refractory per cubic foot of firebox, the ceramic refractory configured into a crucible shape.
 20. The method of claim 18 wherein the high-thermal-mass hydronic furnace further includes a draft air-flow system connecting the firebox to the environment external to the insulated outer casing. 